High temperature coating for silicon steel



United States Patent 3,1693% HHGH TEMPERATURE COATlNG FOR SllLlCGN William H. Schaefer, In, and .lanies K. iltanley, Pittsburgh,

Pa, assignors to Crucible Steel Eompany of America,

Pittsburgh, Pa, a corporation of New .lersey No Drawing. Filed May 18, 1am, Ser. No. 29,834 21 Claims. (Cl. Mid-63) This invention relates to the production of insulative coatings for ferrous metal stock and more particularly, to refractory coating compositions and silicon steel articles coated therewith exhibiting improved electrical properties. The invention also relates to the use of insulative coatings for the purpose of improving the electrical properties of silicon steel and for the purpose of removing impurities from silicon-bearing ferrous metal stock.

As is well known, energy losses in the cores of trans formers, generators and the like are reduced by constructing the cores from thin lamination of silicon steel. The thin laminations, being insulated from each other, materially reduce core losses due to eddy currents; and the addition of silicon to the steel gives the characteristics of high permeability and low hysteresis loss, as well as lower eddy current losses due to high electrical resistance.

The core loss in watts per pound at 60 cycles and 15 kilogausses is the basis for grading silicon steel sheets. Practically all elements other than silicon, when added to iron, adversely afifect the core loss of the material. Accordingly, to develop the optimum magnetic properties of iron-silicon alloys, particularly the grain oriented types,

the undesirable impurities such as carbon, sulphur, nitrogen, and oxygen in the form of inclusions must be reduced to the lowest possible values. It is common practice in the art to reduce these undesirable elements at whatever stages of manufacture it is possible to do so. Iron-silicon alloys are usually melted by the basic open-hearth method, cast into ingots, slabbed, hot reduced, cold reduced, annealed, again cold reduced, decarburized, and then hightemperature annealed. As far back as the open-hearth charge, an effort is made to carefully select materials containing as low a sulphur and alloy content as possible and to minimize elements, such as aluminum, which form very stable oxides. During melting, the quantities of carbon and sulphur in particular are reduced.

A special effort is made to remove carbon by a do carburizing anneal when the strip is at gauges of 0.012 of 0.014 inch or thinner. During decarburization, the silicon sheets are open annealed at a temperature of about 1500 to 1600 F. in the presence of hydrogen and water vapor, substantially as described in US. Patent No. 2,385,332, issued in the name of Carpenter et al. as inventors. The water vapor effects rapid oxidation of the carbon in the silicon steel sheet while the reducing characteristics of the hydrogen prevent oxidation of the iron.

Prior to high-temperature annealing of silicon steel, which usually is in the form of thin laminations or coiled sheet having a thickness varying from about 0.061 to about 0.014 inch, a coating of refractory material must be applied to the steel. This coating is intended to serve a number of purposes not the least of which is the prevention of self-welding of the laminations or coils during the high-temperature anneal, which usually occurs at a temperature in the range of about l850 to 2250" F. The coating is also intended to permit hydrogen, which comprises the annealing atmosphere, to diffuse readily into the steel. This is believed to promote the removal of impurities which form compounds on the surface of the steel by combination with said hydrogen. The coat- 5 ing is further intended to provide electrical insulation between the laminations or coils in the final products, e.g., a transformer.

In practice, magnesia is the most commonly used refractory coating material. However, neither this material nor others disclosed in the prior art satisfactorily accomplish all of the aforementioned intended purposes. Magnesia does not provide an extraordinarily high electrical resistance. As a result certain electrical applications of silicon steel, e.g., power transformers, require an additional coating of electrically insulative material.

This is true also of dolomite, which requires said additional coating where the silicon steel is intended for both distribution and power transformers.

Accordingly, a principal object of the invention is to provide a coating composition characterized by increased electrical resistance.

A second object of the invention is to provide a mixture of refractory material for use in coating silicon-bearing ferrous metal stock and for improving the electrical properties thereof.

Another object of the invention is to provide an article of manufacture exhibitiiig improved electrical properties comprising silicon-bearing ferrous metal having on its surface alightly adherent insulative refractory coating.

S till aiiother object of the invention is to provide a 'pfocess for reducing the core loss of silicon steel stock by the application of a tightly adherent insulative refractory coating to the surface thereof.

Yet another object of the invention is to provide a method for removing impurities from silicon-bearing ferrous metal while applying to the surface of same a coating to prevent adhesion between laminae thereof during a high temperature anneal.

A further object of the invention is to provide a process for the production of silicon steel stock exhibiting improved electrical properties and having on its surface a tightly adherent insulative coating.

Other objects will be apparent from the following description of the invention, a preferred embodiment of which, illustrative of the best mode of practicing the invention, is set forth therein:

Preliminary investigations of many possible high-temperature coatings for silicon steel showed that mixtures of chromic oxide (Cr O and calcium oxide (CaG) had promise, and that an addition of up to about 5% barium oxide (BaO) improved the resulting electrical properties. A specific mixture of about chromic oxide and about 45% calcium oxide was prepared in powder form and added to distilled water to form a slurry. The slurry was thoroughly agitated in a Waring Blendor and then 0 applied to the surface of semiprocessed silicon steel. Epstein strips (see Metals Handbook, 1948 pages 588 and 589) coated thereby and control strips coated with magnesia were annealed at temperatures of 1950, 2050 and 2l50 F. for periods of 1, 5, 10 and 25 hours, in an atmosphere of dry hydrogen (below minus 20 F. dew point).

Tests were run to determine comparative core loss values and electrical resistance values. The average 5 values obtained by preparing bundles of 24 Epstein strips 3,160,509 3 each, selecting 5 strips from each bundle and making 3 TABLE II readings per strip are given in Table I. Test Results of 70% Cr O 30% CaO Coating Annealing Temperature of 2150 F.

TABLE I ,5 Core Loss Electrical Resistance Test Results 55% Cr O -45% CaO Coating p g (Watts/Dorm films/Se cm) (How) I O 0 Core Loss Elec. Resis. Mgo 7O 3 ,?6 A g0 70 aO Annealing Annealing (watts/pound) (ohms/sq. cm.)

i s (ti our 662 7. 4 20, 000 Mgo 55 CF20? Mgo 55 one? 10 i0 665 3.0 100 000 CaO CaO n l 0 04 764 It is evident from the Table II results that a significant 5 :6 81 improvement in electrical properties is obtained with the it) 0- O lower-lime mixture of 70% Cr O 3-0% CaO.

i 8& 0:674 1007000 During the course of the present investigation chromlc 3 g- ;37 0- 8 6. 0 0,000 oxide-calcium oxide mixtures of varying proportions rang- 25 mg 8: 31% ijgg ggg ing from 100% chromic oxide to 100% calcium oxide 722 0.2 52 were prepared with additions of up to 5% barium oxide. 10 g 8 3? 3 g 2 11y88:gg8 2O Epstein strips were coated with a number pf these mixes 0. 622 0.672 and then annealed at temperatures of 2050 F. and 2150 F. for a period of ten hours in a dry hydrogen atmosphere. Test data for core loss and electrical resistance determinations thereof are compared with those for magnesia in Table III.

1 Range of a total oi readings.

Z Twentyeight out oi 30 readings were infinity.

TABLE III Test Results of Cr O -CaO-BaO Coatings Composition (Parts by Weight) Annealing Electrical Resistance Temp. Core Loss Resistance 2 Infinity F.) (watts/lb.) (ohms/sq. cm.) Readings 1 (H 0 0210 BaO Mg'O 1 Total of 15 readings per Epstein bundle. Average of readings other than infinity.

It will be noted from Table I that the core losses attendant the use of 55% Cr O 45% CaO coating are comparable to those attendant the use of the prior art mixtures results in improved core loss values with annealmagnesia coating for annealing temperatures of 1950 ing temperatures of 2050 F. and 2150 F. This improveand 2050 F. However, the electrical resistance values ment is also achieved by the addition of 5 parts BaO to of the 55% Cr O -45% CaO coating for the annealing the 70% Cr O 30 %Ca0 mixture when an annealing temperature of 2050 F. are markedly superior. For the temperature of 2150 F. is employed. In all cases Table annealing temperature of 2150 F. electrical resistance III shows that the addition of 5% BaO to C1' O -CaO values of the 55% Cr O 45% CaO coating are superior mixtures results in superior electrical resistance. although core loss values are somewhat inferior. In addition to producing coating compositions exhibit- In an effort to improve core loss values at the annealing ing improved electrical properties, the mixtures of the temperature of 2150 F. the percentages of ingredients in present invention produced smooth slurries and was unthe 55% Cr O -45% CaO mixture were varied to attended by the problem of thickening which normally Cr O 30% CaO on the theory that the lime fraction was accompanies magnesia. It was noted that application of too reactive at the high temperature. 70 the inventive mixtures and slurries to Epstein strips by In accordance with the aforementioned procedure Eprolling produced coatings of pronounced uniformity. stein strips coated with a mixture of 70% Cr O 30% Other properties such as fired uniformity, adhesion and CaO were annealed at 2150 F. for period of 5 and 10 resistance to cracking and spalling on bending were obhours and tests for core loss and electrical resistance were served to be equal or superior to magnesia. made, the results of which are shown in Table II. Having thus described our invention so that others It is significant to note that the addition of 5 parts R210 to the 55% Cr O 45% C210 and 45% Cr O -55% CaO skilled in the art may be able to understood and practic the same, we state that what we desire to secure by Letters Patent is defined in what is claimed.

We claim as our invention:

1. A process for reducing the core loss of silicon steel stock having on its surface a tightly adherent insulative coating which comprises coating the surface of said stock with a slurry of ceramic material containing between about to 85% chromic oxide, the balance calcium oxide, and heat treating said stock whereby said surface fuses with a portion of said ceramic material to form a glassy layer.

2. In the method of treating silicon-bearing ferrous metal stock to remove impurities While applying to the surface of the stock a coating to prevent adhesion between laminae thereof during a high temperature anneal, the steps comprising applying to the surface of the stock a coating of ceramic material containing about 15 to 85% chromic oxide, the balance calcium oxide, and subjecting the coated stock to a high temperature anneal whereby said ceramic material reacts with the impurities in the stock to form compounds on the surface thereof permitting greater diffusion of the impurities from the center of the stock to said surface While forming an insulative coating thereon.

3. In a process for the production of silicon steel stock exhibiting improved electrical properties and having on its surface a tightly adherent insulative coating the steps comprising coating the surface of said stock with a mixture of refractory material containing about 15 to 85% chromic oxide, the balance calcium oxide, and subjecting the coated stock to a high temperature anneal whereby said refractory material reacts with the impurities in the stock to form compounds on the surface thereof permitting greater diffusion of the impurities from the center of the stock to said surface while forming a glassy layer thereon.

4. An article of manufacture for use in electrical applications comprising silicon steel sheet or strip which has been subjected to a high temperature anneal after having had applied to its surface a slurry of ceramic ma terial containing 15 to 85% chromic oxide, the balance calcium oxide.

5. An article of manufacture for use in electrical applications comprising silicon-bearing ferrous metal having on its surface a tightly adherent insulative coating obtained by applying thereto a mixture of refractory material containing 15 to 85% chromic oxide, the balance calcium oxide, and annealing.

6. An article of manufacture comprising silicon steel having on its surface a glassy layer obtained by applying thereto a mixture of refractory material containing 15 to 85% chromic oxide, the balance calcium oxide, and annealing in the range of 1850" to 2250 F.

7. An electrical coil according to claim 6.

8. A transformer coil according to claim 6,

9. A mixture of refractory material consisting essentially of 15 to 85 parts by weight chromic oxide, 85 to 15 parts by weight calcium oxide, and from an effective amount up to about 5 parts by weight barium oxide for the purpose of improving core loss in silicon steel.

10. A slurry for use in coating silicon-bearing ferrous metal stock, comprising a dispersoid consisting essentially of 15 to 85 parts by weight chromic oxide, 85 to 15 parts by weight calcium oxide and up to about 5 parts by weight barium oxide, and a liquid vehicle in sufficient amount to enable the slurry to be applied as a coating to the surface of the metal stock.

11. A coating composition for use in the prevention of self-welding of silicon-bearing ferrous metal stock, said composition consisting essentially of about 15 to 85 parts by weight chromic oxide, about 85 to 15 parts by weight calcium oxide and from an effective amount up to about 5 parts by weight barium oxide for the purpose of improving electrical resistance.

12. A coating composition according to claim 11 wherein the chromic oxide content is about 70 parts by Weight.

13. A coating composition according to claim 11 wherein the chromic oxide content is about 45 to parts by Weight.

14. A coating composition according to claim 11 wherein the chromic oxide content is about parts by weight and the calcium oxide content is about 30 parts by weight.

15. A coating composition according to claim 13 where in the calcium oxide content is about 55 to 45 parts by weight.

16. The process of claim 1 wherein said ceramic material includes up to about 5 parts by weight barium oxide.

17. The process of claim 2 wherein said ceramic material includes up to about 5 parts by weight barium oxide.

18. The process of claim 3 wherein said ceramic material includes up to about 5 parts by weight barium oxide.

19. An article of manufacture for use in electrical applications comprising silicon-bearing ferrous metal having on its surface a tightly adherent insulative coating consisting essentially of about 15 to parts by weight chromic oxide, about 85 to 15 parts by weight calcium oxide, and up to about 5 parts by weight barium oxide.

20. An article of manufacture according to claim 19 wherein the chromic oxide content is about 45 to 55 parts by weight and the calcium oxide content is about 55 to 45 parts by weight.

1 21. An article of manufacture according to claim 19 wherein the chromic oxide content is about 70 parts by weight and the calcium oxide content is about 30 parts by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,231,024 Pole Feb. 11, 1941 2,271,362 Field Jan. 27, 1942 2,843,507 Long July 15, 1958 

10. A SLUFFY FOR USE IN COATING SILICON-BEARING FERROUS METAL STOCK, COMPRISING A DISPERSOID CONSISTING ESSENTIALLY OF 15 TO 85 PARTS BY WEIGHT CHROMIC OXIDE, 85 TO 15 PARTS BY WEIGHT CALCIUM OXIDE AND UP TO ABOUT 5 PARTS BY WEIGHT BARIUM OXIDE, AND A LIQUID VEHICLE IN SUFFICIENT AMOUNT TO ENABLE THE SLURRY TO BE APPLIED AS A COATING TO THE SURFACE OF THE METAL STOCK. 